Coupling system



COUPLING SYSTEM Original Filed Sept. 26. 1927 5 Sheets-Sheet l ZJI,

INVENIOR [0760/77 War/55 l /1 BY r U WJ M ATTORNEYS Jan. 9, 1934. WALSH1,942,786

COUPLING SYSTEM Original Filed Sept. 26. 1927 5 Sheets-Sheet 2 INVENTORATTORN EYJ Jan. 9, 1934. L. WALSH COUPLING SYSTEM Original Filed Sept.26. 1927 5 Sheets-Sheet 3 ATTORNEYS Jan. 9, 1934. L. WALSH 1,942,786

COUPLING- SYSTEM Original Filed Sept. 26, 1927 5 Sheets-Sheet 4 INVENTORl [2760M M aZJf;

ATTORNEYS Cum 0 m 0%? cum oum 26, 1927 5 Sheets-Sheet 5 I INVENTORZ0760! Wa/Jfi BY @4441, I I ATTORNEY5 L. WALSH COUPLING SYSTEM OriginalFiled Sept.

Jan. 9, 1934.

Patented Jan. 9, 1934 UNITED STATES PATENT OFFICE COUPLING SYSTEMLincoln Walsh, Elizabeth, N. J.,

Hazeltine Corporation, Jersey City, N.

assignor to J a June 22, 1932. Serial No. Canada December 14,

9 Claims.

The present invention relates to radio-frequency vacuum tube amplifiersof the transformer-coupled type and, more particularly, tuned amplifiersintended to operate over a fairly wide range of frequencies, as forexample, in broadcast receivers. This application is a division of myoriginal application Serial No. 222,009, filed September 26, 1927.

In the design of tuned radio-frequency amplim fiers one of thecontrolling factors to be dealt with is the tendency toward excessiveregeneration and consequent oscillation which is due mainly to thecouplin capacity between the grid and plate of each vacuum tube. Somepro- 1 vision must always be made against this tendency, preferably byneutralization. But since neutralization involves the use of a fixedneutralizing capacity which is balanced against the coupling capacity ofa vacuum tube and since individual vacuum tubes, even of the same typeand manufacture, are bound to vary to a certain extent, it follows as adirect result that there is a limit to the amplification per stage atany given frequency beyond which it is impracticable to go. Since theinter-electrode (plate-grid) capacitive reactance decreases with risingfrequency, thus allowing a greater amount of high frequency energy to befed back from the: plate circuit to the grid circuit of each tube, thepermissible amplification per stage also decreases with risingfrequency. As a matter of experience,

the permissible amplification per stage at 1500 kilocycles per second isvery much less than at 500 kilocycles per second. These frequencies arethe limits of the broadcasting band as now estab lished. Theory andexperience both indicate that the amplification should vary inversely asthe square root of the frequency in order that adequate manufacturingtolerance in the vacuum tube coupling capacity may be allowed.

On the other hand, the amplification per stage obtainable with couplingtransformers cf the conventional type, that is, transformers havingfixed coils of high step-up ratio and low losses,

is less at the lower than at the higher frequencies.

As between the frequencies 500 kilocycles per second and 1500 kilccyclesper second the actual amplification per stage is very much less at thelower frequency. Thus, while the permissible amplification per stage atthe lower frequency end of the broadcast band is much greater than atthe high frequency end, the characteristics of the conventionalradiofrequency transformer are such that the actual amplificationobtain- 222,009. Divided and this application 618,681, and in 1927 ableis very much less at the low frequencies than at the high frequencies.

11; has sometimes been attempted to modify the amplificationcharacteristics of the conventional transformer to increase theamplification at low frequencies without also increasing theamplification at high frequencies. This can be done by introducingsources of energy loss which are mainly effective at high frequencies,but this expedient has the serious disadvantage of impairing theselectivity at high frequencies.

Not only is the lower-frequency amplification with conventionaltransformers limited by the instability which would occur at higherfrequencies, but the conventional transformer arrangement does notpermit of high amplification at low frequencies irrespective ofinstability. The reason is that in order to be able to tune to allfrequencies within the broadcast band it is necessary to employ a tuningcondenser having relatively great capacity at the lower frequencies.Such a large capacity in combination with transformer coils ofconventional small dimensions and such as will give satisfactoryfidelity, introduces an excessive secondary conductance at lowfrequencies. The high secondary conductance thus introduced has theeffect of reducing the amplification at low frequencies, as may beeasily demonstrated both mathematically and by actual performance tests.

The primary object of the present invention is to provide aradio-frequency amplifier from which may be obtained, at all frequencieswithin its range, the highest amplification consistent with stability,as limited by the customary tolerance in coupling capacities of vacuumtubes, without impairing the selectivity at high frequencies (where thisis naturally the poorest), or the fidelity at low frequencies (where thelatter characteristic is naturally the poorest).

Another object, in furtherance of the primary invention, is to provide astructural and electrical arrangement which is adequately simple andtrouble-proof in operation.

The present invention accomplishes these ends by providing means adaptedto change simultaneously the voltage ratio of the radio-frequencycoupling systems, the self-inductance of the tuning inductance and thetuning capacity, whereby the maximum permissible degree ofamplification, or any desired approximation thereto, is obtained at allfrequencies at which the receiver is designed to operate.

Considering a radio-frequency coupling system of the conventional typecomprising a fixed-ratio transformer whose primary is included in theplate circuit of an amplifying vacuum tube and whose secondary windingtogether with a tuning condenser is connected in the grid-filament(input) circuit of a succeeding vacuum tube, the greatest amplificationwould occur at the highest frequency to which the receiver may be tuned,and the least amplification at the lowest frequency. Therefore, it isordinarily necessary to design such transformers so as to avoidexceeding the permissible amplification per stage at the highestrequency, as limited by the tendency toward instability.

In so far as the transformer per se is concerned, the factors whichordinarily determine the degree of amplification are the self-inductanceof the secondary winding, the mutual inductance between primary andsecondary winding and the conductance (or resistance) of the secondarywinding. The voltage ratio, or simply the ratio of a transformer thesecondary circuit of which is tuned is substantially equal to the ratioof the secondary self-inductance to the mutual inductance. In theconventional radiofrcquency transformer now referred to, both thesecondary self-inductance and the mutual inductance remain constantirrespective of the frequency, and these values must, therefore, bedetermined by the requirement that the maximum permissible amplificationmust not be exceeded at the highest operating frequency. If the ratio ofsecondary sel -inductance to the mutual inductance is lowered as theoperating frequency is lowered then the amplification per stage will ingeneral be increased. Furthermore, if the secondary self-inductance isincreased as the frequency is lowered, it will be possible to obtain thedesired amplification while maintaining fidelity and compactness of coilconstruction. The present invention involves the proper adjustment ofthe inductances and the tuning capacity so as to obtain the maximumpermissible amplification at each frequency within the band offrequencies for which the receiver is designed.

Theoretically there are a variety of ways in which this can be done butthe means and method described hereinafter have been found highlysatisfactory from every standpoint and are, accordingly, regarded asrepresenting the preferred embodiment of the invention. In the structureherein described each radio-frequency transformer is provided with amovable metallic shield comprising two cylindrical cup-like partsarranged coaxially, one within the other. The shields or cups, as theymay conveniently be called, are preferably made of a non-magnetic metalof high conductivity, such as copper or aluminum. Each cup is arrangedto slide longitudinally with respect to its associated transformer. Thelarger cup is adapted to variably envelop the outer winding, which isthe secondary, while the smaller cup is adapted to move longitudinallywithin the smaller or primary winding. The effect of moving the metallicshields or cups with respect to the transformer windings is to vary boththe secondary selfinductance and the mutual inductance, each atthedesired rate. The effect of the metal cups or shields disposed closelyadjacent the windings of the transformers is to restrict the crosssection and thereby to increase the reluctances of the magnetic paths,thus reducing the inductances.

The transformers may accordingly be designed with a view to obtainingthe maximum permissible amplification per stage at the lowest operatingfrequency-this value being much higher than that of the permissibleamplification at the highest operating frequency. Then by providingmeans for moving the metal shields or cups so as to increasingly envelopthe transformer windings as the operating frequency is increased theamplification may be decreased to the proper, value for each operatingfrequency.

In the preferred form of this invention the movement of the metal shieldor shields relatively to the transfomer windings is accomplishedautomatically by means of suitable mechanical connections with thetuning control or controls, and preferably the several stages ofamplification are adjusted simultaneously by a single manual control.However, some of the advantages of the invention together with greaterflexibility may be obtained by independently operating the shields andthe tuning condensers.

With reference to the drawings which accompany this specification,

Fig. 1 is a cross sectional view of a coupling unit comprising aradio-frequency transformer with its associated shield together with avariable tuning condenser and a neutralizing condenser, all of which areassembled within a sheet metal receptacle;

Fi 2 is a plan view of the coupling unit shown in Fig. l and is taken asviewed from the same observation point as Fig. 1;

Fig. 3 is a partial elevational view taken along the line 33 of 1 andlooking toward the front of a radio receiver. This figure illustratesthe operating mechanism for the shields and tuning condensers;

4 is a sectional view taken along the line 4-4 of Fig. 1;

Fig. 5 is a plan view of a condenser plate;

Fig. 6 is a comparative amplification graph representing by appropriatecurves the amplification obtainable at different frequencies with aconventional tuned amplifier and with an amplifier in accordance withthis invention; and

Fi 7 is a partial circuit diagram of a neutralized radio-frequencyamplifier adapted for use in conjunction with this invention.

In Figs. 1 and 2 there are illustrated plan views,

Fig. 1 being in section, of a radio-frequency transassociated metalliccup-like former with its shields together with a tuning condenser and aneutralizing condenser-the arrangement shown being in accordance withone of the preferred embodiments of the invention.

.able spacing ring 3 and The transformer a per comprises two coaxialtubes 1 and 2 of secured by bolts 4. 1n this particular case there is,in addition to the primary winding, a neutralizing winding having anequal number of turns. wound on the tube 1. The turns of the primary andneutralizing windings are interleaved. The secondary is wound on thetube 2. The metallic shield structure 5 comprises two coaxialcylindrical cup-like members, or metallic cylinders, 6 and '2 The twocups are secured together forming a unit so that there is created acylindrical slot between the inner cy .ndrical wall of the outermost cupand the outer cylindrical wall of the innermost cup. The tuningcondenser comprises two identical elements, namely, a stator 8 and amovable element 9. Each of these elements has four plates, one elementar as shown. The plates of interleaved with those of the other. Themovable element 9 is rigidly secured to the shield 5 through the mediumof a rigid arm, or connecting member 10 of insulating material, whichmechanically links the metallic shield structure and the movable plates.The shield 5 together with the movable condenser element 9 is adapted toslide axially with respect to the transformer coils. Thus it will beseen that as the plates of the tuning condenser move together--thecapacity of the condenser thereby increasingthe shield 5 is moved to theright. as shown in Fig. 2, whereby it decreasingly affects the magneticfield or path of the transformer causing both the secondaryself-inductance and the mutual inductance to increase. When the cup 6envelops the secondary winding to the maximum extent the reluctance ofthe magnetic field is a maximum, and the secondary self-inductance isconsequently reduced to a minimum. Likewise the mutual inductancebetween primary and secondary windings is decreased as the cup '7 ismoved into closer relation with the primary Winding, that is, to theleft as viewed in Fig. 1. While the two shields interact in theireffects the outer one 6 affects mainly the secondary selfinductance, andthe inner one 7 the mutual inductance. With the proportions indicated inFig. 1of which specific dimensions will be giventhe mutual inductancevaries at a faster rate than the secondary self-inductance, giving thedesired variation in the ratio. Although the arrangement illustratedcomprising two coaxial cups constitutes the preferred embodiment of theinvention, it has been found that similar but less ideal results may beobtained by omitting either .one or the other of the cups 6 and 7. Sincethe function of the shields is to vary the reluctance of the magneticfield it will be apparent that the shield or shields may take a varietyof forms besides the specific arrangement illustrated.

As a part of the tuning condenser structure illustrated in Figs. 1 and 2there is shown a flexible conducting element 11 which is adapted tofunction as one plate of a neutralizing condenser. This flexible platemay be adjusted toward and away from the adjacent fixed plate of thestator 8 by means of the adjusting screw 12. The capacity between theelement 11 and the stator of the tuning condenser may thus be adjustedto a proper value for effecting neutralization.

In order to clarify the illustration, the operating mechanism for thetwo tuning condensers and shields has been omitted from Figs. 1 and 2. Asuitable mechanism for this purpose is illustrated in Figs. 3 and 4. Itwill be realized, however, that the operating mechanism illustrated inFigs. 3 and 4 constitutes only one of a great variety of ways in whichthe same results may be obtained, and that the invention is in nowisedependent upon the particular mechanism by which the operation of thetuning condensers and shields is brought about.

The operating mechanism of Figs. 3 and 4 is arranged for the unitaryoperation of two sets of tuning condensers and shields. It may obviouslybe extended to take care of as many stages of amplification as may bedesired.

Figs. 3 and 4 are views taken along the lines 3-3 and 4-4, respectively,of Fig. 1 and drawn to a somewhat smaller scale than Fig. 1. Aspreviously stated, the operating mechanism shown in Figs. 3 and 4, andnow to be described, has been omitted from Figs. 1 and 2 in order toclarify the disclosure. Referring to Fig. 3, the tuning control knob(not shown) is secured to a shaft 13 on which is fixed a small pinion 14which meshes with a toothed quadrant 15 carried by a shaft 16. Agraduated dial 15a is attached to and moves with the quadrant 15. Alever arm 17, which for convenience is made in the form of an L, issecured to shaft 16 and is rotatable therewith. To the lever 17 ispivotally secured a link 18 which is connected to the member 10 (shownalso in Figs. 1 and 2). Since both the shield 5 and movable condenserelement 9 are attached to the member 10, it is apparent that these partsall move together in'a direction parallel to the axis of the transformerin response to rotation of the tuning control knob. As shown in Figs. 2and 4, the members 10 are secured to a slidable support 19. The latteris, for convenience and rigidity, made of an angle cross-section, and issupported on suitable bearings on which it is free to slidelongitudinally but not otherwise. In Fig. 4 the tuning condenser isshown partially broken away in order that the pivotal connection of thelink 18 to the supporting member 10 may be shown in full lines.

It should be clear from the foregoing description that both of thetuning condensers and shields indicated in Fig. 4 are operated from thesame control knob, and that as many additional sets of condensers andshields as might be desired could be added and operated through the onecontrol. Three stages of tuned radio-frequency amplification have beenfound very suitable.

Incidentally, the present invention lends itself very advantageously tounitary tuning control. This is because of the fact that the tuningcondensers are preferably of small maximum capacity as compared with thetuning condensers ordinarily used in broadcast receivers of theconventional type. The reason why a particularly small tuning condensercan be used to cover a wide band of frequencies is that the tuning isaccomplished by varying the secondary self-inductance at thesame timethat the tuning capacity is varied. Since the tuning condensers may beof unusually small maximum capacity the plates may be heavier and spacedfarther apart without excessive bulkiness. For these reasons it ispossible to maintain a considerably greater precision in manufacturingthe condensers; and the tuning of the several stages of amplificationunder a single control may, therefore, be carried out with greateraccuracy or with less difficulty.

As a specific example, the dimensions of the transformer, shield andtuning condenser shown in Figs 1 and 2 will now be given. The tube 1 130is made of natural formica 1 2 inch outside diameter, 3%; inches long,and inch thick. On this tube are wound a primary coil and a neutralizingcoil. The turns of these coils are interleaved, each coil having 36turns of number 135 38 double-silk-covered copper wire, 16 double turnsper inch. The tube 2 is made of the same material, 2 inches outsidediameter, 3% inches long and inch thick. On this tube is wound asecondary coil consisting of 120 turns of num- 140 ber 25 enameledcopper wire, 48 turns per inch. The cup 6 which forms part of the metalshield is made of sheet copper 0.931 inch thick. Referring to Fig. 1,dimension A is 2%;- inches and B is 2.95 inches. Cup 7 is also made ofsheet 145 copper 0.031 inch thick. Dimension C is 2% inches and D is 1inches.

With the above dimensions, the step-up ratio of the transformer is atall frequencies substantially higher than that giving greatest ampli-15G Fig. 5. The dimensions indicated in Fig. are as follows:

Inches E 3 F 2 G 1 H 1 J K This particular condenser was designed with aView to obtain tuning characteristics lying approximately midway betweenstraight-line-frequency vaiiations and straightdine-wavelengthvariations. The object of doing this is to make the apparent sharpnessof tuning substantially the same at all settings of the tun g controldial. The invention of course, in no sense dependent upon the particularform of tuning condenser which may be used.

With transformers, shields and conde .sers oi the dimensions specifiedabove and in accordance with the design illustrated in 1 and 2 employedin a multistage 'iier using vacuum tubes of the lA type having anamplifie factor of about 8, the results actually obi are indicated bycurve B of Fig. 6. On graph sheet the curve A shows the calculatedmaximum permissible amplification at all quenoies between about 550kilocycles per 0nd and 1550 kilocycles per econd. According to thetheory on which curve A is based, the amplification should varyinversely as the square root of the frequency in order that a practicaliv Au;

respect to the p1ate-grid coupling capacity. is well confirmed by actualexperience. It a be observed that curve B closely follows the form ofcurve A and, therefore, indicates an amplification which variesinversely as the square root of the frequency. In calculating thepermissible amplification at the various frequencies from which curve Awas plotted a tube capacity tolerance of 0.5 microniicrofarad wasallowed. This tolerance represents the permissible manufacturingdeviation of the internal capacity between the plate and grid. It isthis unavoidable deviation which exe cises the greates influence inlimiting the permissible amplification. Curve C,

6, indicates the radio-frequency amplifies.- tion obtainable in a goodrepresentative neutralized receiver having radio-frequency transformersof the general form shown in l but without the metallic shields orequivalent means for accomplishing the purpose. in examination of c e Cit will be at once apparent that the ampliiication at the high-frequencyend of the band is much greater the lowfrequency end. Also by comparisonof curve C with curve A it will be seen that the amplification at thelow-frequency end is very such less than the permissible amplification,v -ereas it closely approaches the maximum permissible amplification atthe high-frequency end.

Curve B indicates how the large discrepancy between permissibleamplification and actual amplification at low frequencies and, in fact,over substantially the entire band of frequencies except for theimmediate region of the upper end, has been corrected by the use ofmetal shields as herein described.

The specific structure which has been described as examplerepresentative of the preferred embodiment of the invention isapplicable for use in the radio-frequency portion of a neutralizedbroadcast receiver of which a partial circuit is shown diagrammaticallyin Fig. 7. This figure illustrates diagrammatically two stages ofneutralized radio-frequency amplification the output of which may bepassed through one or more additional stages of radio-frequencyamplification and thence to a detector or it may be passed directly tothe detector and thence to an audio-frequency amplifier.

In Fig. 7, the three-electrode vacuum tube amplifiers 20 and 21,respectively, are coupled in cascade through the medium or aradio-frequency transformer 22 which may be the transformer shown inFigs. 1 and 2. The primary winding 23 and the neutralizing winding 24 ofthis transformer are interleaved on the same tube, as previouslydescribed in connection with Fig l, and may be in all respectsidentical. One end of the neutralizing coil 24 is connected to aneutralizing condenser 25 the other terminal of which is connected tothe grid of the tube 20. This is in accordance with one form of the wellknown Hazeltine method of neutralization. The secondary winding 26 ofthe transformer 22, Fig. '7, may be the same as specified hereinbeforein connection with the description of Fig. 1. The tuning condenser 27may be identical with that shown in Figs. 1, 2 and 5. The transformer 28which couples the output side of tube 21 with the next succeeding tube(not shown) may be identical with transformer 22 and, of course, thetuning condenser 29 may be identical with the condenser 27. Likewise tis neutralizing condener 30 may be in accordance with the disclosure ofFig. 1 wherein the flexible element 11 comprises one plate of aneutralizing condenser of which the stator of the tuning condenser formsthe other plate.

The present invention has been developed in conjiniction withneutralized amplifiers of the type indicated in general by Fig. '7 andis particularly well adapted for use in connection with that type ofneutralization, but it may be used eiiectively with other methods ofneutralization and oscillation suppression.

I claim:

for moving said structure relative struc" 'i comprising a member of allycon i -ng material having a cylinslot within which said coils may be in-;vhereby structure is adapted to varibases, the outermost cup having aninner cylindrical Wall and the innermost cup having an outer cylindricalwall, said windings being positioned between said walls, and means formoving said cups in a coaxial direction with respect to said windings sothat when said shield is moved said windings are situated at variousrelative positions between said cylindrical walls.

3. A high-frequency transformer comprising cylindrical primary andsecondary windings, a shield for said windings comprising a pair ofmetal cylinders coaxially situated one within the other and electricallyjoined together at one end of each cylinder, said cylinders beinglocated one within and one without said coils, and means for moving saidshield coaxially with respect to said coils so that said coils arevariably enveloped by said cylinders.

4. A radio-frequency transformer comprising a plurality of coils, ametallic structure adapted to be moved co-axially with respect to saidcoils and means for moving said structure relative thereto, saidstructure comprising a member of electrically conducting material havinga slot within which said coils may be inserted, whereby said structureis adapted to variably envelop said coils when moved relatively thereto.

5. In a radio-frequency coupling system, an inductance coil and acondenser having a movable set of plates effectively connected inparallel therewith, a conductive metallic structure comprising acup-like member movable axially with respect to said coil and beingadapted to alter the magnetic path thereof and a rigid arm mechanicallylinking said metallic structure and said movable set of plates, wherebysaid member may be axially moved simultaneously with said movable set ofplates.

6. In combination, an inductance coil and a movable condenser, said coilhaving associated therewith a cup-like metallic shielding member movableaxially with respect to said coil and being adapted to envelop andthereby alter the magnetic path of said coil, said condenser having aset of fixed plates and a set of movable plates movable within andwithout said fixed plates, and a rigid arm mechanically linking saidshielding member and said set of movable plates, the disposition beingsuch that movement of said rigid arm causes said shielding member tomove away from said coil when said movable plates move within said fixedplates, and vice versa.

7. A combination according to claim 6 in which said cup-like shieldingmember comprises a pair of metal cylinders situated one within and onewithout said coil.

8. In combination, an inductance coil and a variable condenser, saidcoil having associated therewith a cup-like metallic shielding membermovable axially with respect to said coil and being adapted to envelopand thereby alter the magnetic of said coil, said condenser having a setof fixed plates and a set of movable plates which are movable within andwithout said fixed plates in the same linear direction as said shieldingmemher, and an arm rigidly linking said shielding member and said set ofmovable plates, and means for moving said shielding member, said movablecondenser plates and said rigid arm as a unit in said linear direction,whereby said shielding member increasingly envelops said coil and saidmovable plates are simultaneously withdrawn from said fixed plates, andvice versa.

9. In combination, a radio-frequency transformer and a variablecondenser, said transformer comprising primary and secondary windingslocated co-axially with respect to each other, a shield adapted to bemoved co-axially withrespect to said windings, said structure comprisinga member of electrically conducting material having a cylindrical slotwithin which said coils may be inserted, said variable condensercomprising "i a group of fixed plates and a group of variable platesadapted to be interleaved between said fixed plates, a rigid arm rigidlyfastened at one end to said shield structure and at the other end tosaid group of movable plates, said arm being movable in the lineardirection which is co-axial with said windings, whereby said shieldstructure and said movable plates are simultaneously moved in the samedirection by motion imparted to said arm.

LINCOLN WALSH.

